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Here's a global map of magnetic declination: enter image description here How can there be zero magnetic declination in places where the magnetic and geographic poles don't line up (which is only at approx. latitude 0 and 180 for the northern hemisphere)? And why are the lines so crooked?

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A pole is a point. If you have two points then there are places where they line up if you'd look towards them.

For the geographic and magnetic poles we use "declination" to describe the angle between the direction towards geographic north versus the angle that the magnetic compass needle points to.

If you are in a place where both line up, then the declination is 0°.

Plotted on map those places form crooked lines because there is geographic variation in Earth's magnetic field.

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  • $\begingroup$ Please read the question carefully before attempting to answer. I made it now clearer with my edit. $\endgroup$
    – new editor
    May 30 at 17:12
  • $\begingroup$ What is wrong with my answer? $\endgroup$
    – bugmenot123
    May 31 at 6:37
  • $\begingroup$ You explained that "there are places where they line up if you'd look towards them". I know that. And that should be only in two lines of longitude - approx. 0 and 180. But on the map there are many different locations where they don't line up which also have zero declination. And that's my question. $\endgroup$
    – new editor
    May 31 at 14:52
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    $\begingroup$ They line up in those places because of variation in the magnetic field as I said. Lining up means the direction of the "magnetic needle" and geographic north here, not against the geographic location of the magnetic pole. $\endgroup$
    – bugmenot123
    May 31 at 19:30
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    $\begingroup$ @bugmenot123 is spot on here - the magnetic field varies geographically and the magnetic dip poles are not symmetrically opposite each other across the globe. Declination is the angle between geographic north and the local direction of the magnetic field, which is the direction in which the horizonal part of the magnetic field vector points at a given location, and not the direction of the magnetic pole from that point. $\endgroup$
    – WJB
    Jun 7 at 13:46
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Consider, instead of looking at an Earth map of Magnetic Declination, looking at a map of the Earth's magnetic field:

enter image description here

Now trace a line across North America in such a way as to intersect the magnetic lines at locations where they are perpendicular to true north. You should end up tracing out the green line in the image posted in the question.

Read on if that sounded confusing: Instead, find and circle the points on the lines in the map of the Earth's magnetic field where the curve of the line is perpendicular to an imaginary line drawn from the true North Poll to that point. When done, the points should all be on the green lines in the map in the posted question above.

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You seem to think that the Earth's magnetic field should be, in some sense, smooth?

It's not.

It approximates to a solenoid field. But it's not generated by electrical currents moving in a solenoid. It's (probably) generated by convection currents driven by density moving conducting fluids through an existing magnetic field (itself generated by the movement of the adjacent cells of the core).

It may be helpful to consider the magnetic fields of Uranus and Neptune. They too are "approximately" solenoidal, and are probably formed by the same process as in Earth (but different materials). But the pseudo-solenoidal field of Uranus is inclined at ~59° to the rotation axis (compared to Earth's ~20°) and is "centred" 1/3 of the way from the core to the south pole. It's no where near symmetrical to the planet. Neptune is worse - it's magnetic "centre" is over half-way from the core to the surface, and not anywhere near the rotation axis. And it's inclined at ~47° to the rotation axis that it's not on.

Compared to those, Earth's field is reasonably well behaved. The biggest (and most variable, within human lifetimes) anomaly is the "South Atlantic Anomaly", which may indicate the location of a rising plume in the core. But that's far from agreed.

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